Marine seismic detonator



March 4; 1969 E. M. PATTERSON MARINE SEISMIC DETONATOR Filed Feb. 24, 1967 firth/Ive I Av 4.

United States Patent 3,430,566 MARINE SEISMIC DETONATOR Edward Mervyn Patterson, West Kilbride, Scotland, as-

signor to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain Filed Feb. 24, 1967, Ser. No. 618,413 Claims priority, application Great Britain, Mar. 18, 1966,

11,988/66 US. Cl. 102-28 6 Claims Int. Cl. F42b 37/00 ABSTRACT OF THE DISCLOSURE This invention relates to an electric detonator for use with explosives in marine seismographic prospecting.

In marine seismographic prospecting, charges of explosive comprising a main explosive charge, booster and electric detonator are tired underwater to generate seismic pulses. Occasionally, a charge breaks away from the firing line and becomes lost in the sea. The detonators which have been used hitherto, containing an electric fusehead and sensitive explosive charges sealed by a neoprene plug within a metal casing, can remain live underwater for several weeks before the casing becomes corroded and punctured to allow the water to desensitise the fusehead and explosive charges. There is, therefore, a danger of a lost live detonator being washed up on the shore or dragged up from the sea. In British Patent No. 1,033,793 a seismic detonator was proposed which had an aperture in the casing sealed with a plug of water-destructible resilient material extending through the aperture. In use the plug resisted the ingress of water for a period to allow the detonator to be fired but was eventually destroyed by the water to allow the explosive charges to be desensitised. The preferred plug was a moulded plug of watersoluble thermoplastic material located in the mouth end of the casing around the electrical leading wires. This plug has not proved to be ideally suitable because the time required for the disarming of the detonator could not be sufliciently well controlled. A detonator which was disarmalble by immersion in water has also been proposed in United States Patent 1,901,469 for use in coal mines. This detonator had a perforated casing wherein the perforations were closed by a film of gelatine. This type of closure is not, however, satisfactory for a marine seismic detonator because a film made from a colloid which has been completely dissolved and dried is permeated by water only very slowly. If the film is made sufficiently thin to allow a sufficient rate of water permeation, it is mechanically weak and ruptures, thus allowing the detonator to be destroyed immediately on immersion in water before the normal tiring can take place.

It is an object of the present invention to provide a detonator for marine seismographic prospecting operations which on immersion in water will resist the ingress of water for sufficient time to allow the detonator to be fired and which will thereafter become desensitised within a predeterminable period.

In accordance with this invention a seismic detonator suitable for marine use has its electric fusehead and explosive charges enclosed within a casing having an aperture obturated with particulate water-gellable colloid.

Patented Mar. 4, 1969' In a preferred form the detonator casing has one or more apertures in the normally blind end of the detonator casing which contains the explosive charges, and the, or each, aperture is covered with a layer of particulate watergellable colloid, the explosive charges being positioned in the casing above said colloid.

Examples of suitable water-gellable colloid include gela tine, polyvinyl alcohol, maleic anhydride/vinyl copolymer, and water-soluble cellulose ethers. The water-soluble cellulose ethers such as, for example, sodium carboxymethyl cellulose (SCMC) and methyl cellulose 'are especially advantageous because of the ability of granules of the material to bind together when pressed into the detonator casing.

The size of the particles of water-gellable colloid may vary over a wide range but colloids of average particle size in the range 16 to 60 b.s.s. are convenient and are easily loaded into the usual size of detonator casing.

When the detonator of the invention is immersed in water, the granules of the gellable materials which come into contact with the water become swollen and gelled and the resulting gel retards further water penetration, thus providing a delay period during which the detonator can be fired reliably. The water continues to permeate the layer of gel and eventually sufficient water penetrates to the explosive charges and the fusehead to disarm the detonator. The rate permeation increases with increasing water pressure so that any detonator which sinks is disarmed at an increased rate in proportion to the depth of immersion. The gelled layer is relatiely weak and deformable under hydrostatic pressure so that a sunken detonator will be disarmed more quickly under hydrostatic pres sures which are sufficiently high to deform the gelled layer.

The disarming time can be controlled by variation of several factors including the aperture size and the chemical and physical nature of the obturating water-gellable material. For example, the layer thickness, viscosity and degree of pressing of the water-gellable material covering the aperture all affect the disarming time.

One preferred construction of detonator, which includes further features of the invention, will now be described, byway of example only, with reference to the single figure of the accompanying drawing which shows a longitudinal section of the detonator.

The detonator comprises an elongated cylindrical tubular casing 11 of an aluminium/manganese alloy having a circular aperture 12 in its end 13. The casing end 13 contains a layer of particulate water-gellable material 14 immediately over the aperture 12, a base charge 15 of pentaerythritol tetranitrate (PETN) and a priming charge 16 of lead azide, all the contents being consolidated by pressing. An electric fusehead 17, comprising a bridgewire heating element 18 embedded in a matchhead of defiagrating composition 19 and connected to two insulated connecting wires 20, is located within the casing 11. The wires 20 extend through a mouth end of the casing 11 and are located therein by a neoprene plug 21 around which a portion of the mouth end of the casing is firmly crimped.

The principles and practice of the invention are further illustrated by the following specific examples of certain preferred embodiments.

In all the examples the detonator casing 11 had an internal diameter of 0.25 in., the base charge was 0.8 g. PETN, the firing charge was 0.15 g. lead azide priming composition and the aperture 12 was 0.03 in. in diameter. The details of the layer of water-gellable material 14 and the performance of the detonators when immersed in water at a depth of 8 feet are given in the accompanying Table 1. The particle size of the water-gellable material was 16-60 b.s.s. in all cases.

mersion of said detonator and to allow the particles which come into contact with the water to become swollen and It will be noted from the results in Table 1 that the disarming time was longer for SCMC than for gelatine and this was attributed to the better initial cohesion of the SCMC. The results also show that the disarming time increases with increasing viscosity and quantity of SCMC.

Results showing the efiect of water pressure on the disarming times of Examples 4 and 5 detonators are given in Table 2. The immersion time was 1 hour.

TABLE 2 Pressure Exam le 4, No. Example 5, No.

tiring/bib. Tested Firing/N 0. Tested What I claim is:

1. A seismic detonator for marine use comprising a casing containing an electric fusehead and an explosive charge and having an aperture therethrough obturated with water-permeable time-delay sealing means for permitting permeation of water into said casing after a period of immersion and having a water-permeation rate which increases with an increase in Water pressure, said sealing means including a porous layer of pressed particulate water-gellable colloid obturating said aperture, the particles of said layer being consolidated as a result of being pressed, said layer being sufliciently thick and sufficiently pressed to prevent immediate ingress of water upon imlose ether.

3. A detonator as claimed in claim 2 wherein the watergellable colloid comprises sodium carboxymethyl cellulose or methyl cellulose.

4. A detonator as claimed in claim 1 wherein the Watergellable colloid has an average particle size in the range 16 to 60 b.s.s.

5. A detonator as in claim 1 wherein said detonator casing has a normally blind end containing the explosive charge, said normally blind end having at least one aperture therethrough, said particles of water-gellable colloid being consolidated in the form of a layer which is positioned in the casing between said explosive charge and said aperture.

6. A detonator as in claim 5 wherein said particles of water-gellable colloid are consolidated in the form of a layer by having been pressed into said casing and wherein said layer extends substantially completely across the transverse inner dimension of said casing.

References Cited UNITED STATES PATENTS 3,279,372 10/1966 Patterson 10228 3,322,066 5/1967 Griffith et al. 102--24 VERLIN R. PENDEGRASS, Primary Examiner. 

